Modern jet aircraft and other air vehicles require accurate measurements of the Outside Air Temperature (OAT) for inputs to the air data computer and other airborne systems. An important component of the OAT is the Total Air Temperature (TAT). The TAT is the ambient temperature plus the temperature rise resulting from adiabatic heating caused by the slowing down or stopping of the air fluid at the measuring device (i.e., the maximum air temperature attainable by 100% conversion of the kinetic energy from the relative motion of probe and fluid).
Probes utilizing tubular bodies to reduce the velocity of and measure the temperature of a sample from an air stream moving within the tube have been developed to adequately measure TAT, see, e.g., U.S. Pat. Nos. 2,588,840 and 4,152,938. A strut positions the probe away from the aircraft to enable it to be immersed in air thermally uninfluenced by the aircraft's immediate exterior. Probes of this type implement insulation means about the temperature sensor device (measurement transducer) in an attempt to thermally isolate the measured sample. However, this type of probe encounters several problems. First, boundary layers of stalled air accumulate within the tube's sample chamber, thereby thermally contaminating the measured air. Also, the sensor within the chamber can be detrimentally exposed to particulates within the airstream. In addition, under certain conditions, air moisture causes ice to build up within the probe, causing temperature measurements to be inaccurate.
To address the ice build-up problem, heating apparatus is used to inhibit ice formation within the probe. Boundary layer reducing means can also be added to limit the size of the boundary layer, thus reducing mixing of boundary layer air with the measured air sample. Furthermore, probes have been designed with a sampling chamber for housing the sensor, reachable only by a portion of the airflow that is diverted to take an abrupt turn away from the path of the entering airflow. This configuration protects the sensing element against flying particulates, see, e.g., U.S. Pat. Nos. 2,970,475 and 5,302,026.
Many such prior art probes utilize an elongated transducer or thermal sensor with a cylindrical sensing surface. The sensor is positioned with its longitudinal axis parallel to the diverted airflow. This has generally been done by locating the longitudinal axis of the sensor parallel to the longitudinal axis of the sensor strut and diverting air into the strut. See, e.g., U.S. Pat. No. 5,302,026. However, such configurations require significant volume for housing the sensor and surfaces handling the diverted airflow, presenting several disadvantages. An increased number of heating devices must be utilized to adequately prevent ice from forming within the extensive interior surfaces of the probe's air flow pathways. In the typical heating arrangement such heating devices must be installed with significant hand labor, increasing the cost of the device. Moreover, to yield accurate measurements with relatively fast response times, it is necessary to use relatively complex sensor designs that are hollow or have fins or other special heat transfer arrangements, leading again to increased labor and/or materials expense.
In one heating arrangement known in the prior art, heating is provided by wrapping coaxial cable into grooves formed in the surfaces to be heated. This wire is covered and secured in place in a brazing operation. The resulting surfaces need to be dressed to make them aerodynamically smooth. Installing this form of heating can add significantly to the cost of a finished temperature sensor product.
Accordingly, what is desired in the art is a temperature probe capable of operating accurately and effectively in extreme environments that is configured to reduce the overall size of the probe module, to thereby reduce expense, as well as to improve heating efficiency.